The present invention relates to a line head for improving a defect rate of the line head by preventing print quality from being deteriorated even when a pixel defect is present in a light emitting element and an image forming apparatus using the same.
Conventionally, an image forming apparatus using an electrophotography method, such as a copier, a printer, a facsimile or the like generally includes a laser scanning optical system as an optical writing device.
In recent times, there is developed an image forming apparatus using a record array head aligned with a plurality of columns of optical record elements as optical writing device. For example, JP-A61-182966 discloses a record array head in which image data is irradiated to a sensitized drum while shifting the image data in a sub scanning direction. Therefore, an image can be formed at high speed even when an optical record element having a low light emitting output.
Further, JP-A-64-26468 discloses that high speed formation is dealt with by increasing a light emitting amount by using an El element panel aligned with a number of EL elements and making image data flow to the EL element panel at speed equal to speed of moving a sensitized drum. Further, JP-A-11-129541 discloses that when one pixel is multiple recorded by using two columns or more of array chips, gray scale control is carried out by changing a number of light emitting elements to be light emitting.
However, according to a line head having a plurality of light emitting elements in this way, owing to a problem in view of fabricating steps, there is a pixel defect which is not light emitting or a light emitting amount of which is extremely small by a certain probability. For example, when a plurality of columns of light emitting elements are arranged, there is a defect in the light emitting element is 20%. Table 1 shows a calculation of defect rate when a necessary light amount is provided by emitting only one column of light emitting elements in the case that one column of the light emitting elements is arranged in a line head.
TABLE 1SystemOne column headDefect rate80.0%
In Table 1, when two columns of light emitting elements are arranged in a line head and one column thereof is made to be light emitting, since a probability that a pixel defect is present in the column of the light emitting elements is 20%, the defect rate becomes 80% (1-0.2=0.8).
Table 2 shows an example of providing a necessary light amount by carrying out multiple exposure when the defect probability is set to 20%. In this example, there are used two columns of light emitting elements in columns of light emitting elements arranged in a line head by a plurality of columns thereof. In Table 2, when two columns of light emitting elements are arranged in the line head, the defect rate of one column of the light emitting element column is 80% from Table 1 and therefore, the defect rate in this case is 0.8×0.8=0.64, that is, 64%.
TABLE 2SystemTwo column headDefect rate64.0%
Further, Table 3 shows the defect rate when a plurality of light emitting elements are arranged in a line head and multiple exposure is carried out by using three columns of light emitting elements. In this case, when the defect probability is set to 20%, the defect rate becomes 0.83=0.512, that is, 51.2%. As shown in Table 1 through Table 3, when a number of columns of light emitting elements is increased, the defect rate of the line head is reduced.
TABLE 3SystemThree column headDefect rate51.2%
In this way, when the pixel defect is brought about in the light emitting element, there poses a problem that light emitting amounts of the respective light emitting elements differ from each other. Generally, there is a case in which light emitting amounts of light emitting elements differ from each other by a problem in view of fabrication or the like and therefore, in JP-2000-127492, uniform formation of light emitting amounts of respective light emitting elements is achieved by adjusting light emitting areas.
In this way, according to a line head using a plurality of columns of light emitting elements, when a number of pixels is increased, a probability that a pixel defect is present is increased. FIGS. 17A and 17B illustrate explanatory views showing an example of such a pixel defect. In FIG. 17A, there is a pixel defect at a light emitting element Gx in a light emitting element column L0 arranged at a line head. When columns of pixels of Aa through An are scanned and exposed as shown in FIG. 17B by a light emitting element column L0, a white streak extended in a vertical direction as designated by notation Bx is brought about in a print image, print quality is significantly deteriorated and a total of the line head cannot be used.
FIGS. 18A and 18B illustrate explanatory views showing a second example of pixel defect. According to an example shown in FIG. 18A, when columns of light emitting elements of La through Lc are arranged in a line head, there is present a pixel defect at a light emitting element Gy of the light emitting element column Lb. An arrow Y direction of FIG. 18B is defined as a main scanning direction and an X direction is defined as a sub scanning direction. When multiple exposure is carried out while moving an image carder in the sub scanning direction by using the line head of FIG. 18A, as designated by notation By of FIG. 18B, a print image is formed with a portion thinner than other region and image quality is deteriorated.
In this way, according to a line head using a plurality of light emitting elements, it is a serious problem that print quality is deteriorated by deterioration of defect rate by a pixel defect. Further, also when a pixel defect is produced by a deterioration in durability in using a line head, the print quality is significantly deteriorated similarly to thereby pose a problem that a total of the line head needs to replace.
Particularly, in an image forming apparatus applied with a multiple exposure system, a number of light emitting elements is larger than that in the a normal system and the probability that the pixel defect is present is increased. Therefore, there poses a problem that a defect rate is further deteriorated or replacement of the line head owing to the deterioration in durability is increased. In the case of the multiple exposure system, more or less dispersion of luminescence is averaged so as to cancel. However, since the pixel defect is different from other light emitting element in the light emitting amount significantly, an unallowable deterioration of image quality is brought about.
Further, there poses a problem of complicating fabricating steps by achieving uniform formation of the light emitting amount of each light emitting element by adjusting the light emitting area as in JP-A-2000-127492.